Air pressure within an aircraft cabin is controlled during ascents and descents as a function of: ambient pressure, aircraft cabin pressure and the pressure at the aircraft landing site. Control is typically performed by an automatic cabin pressure controller.
As described in U.S. Pat. No. 3,473,460 to Emmons, and assigned to the assignee of the present invention, an aircraft is designed to withstand a maximum pressure differential between the cabin and ambient pressures. The '460 patent discloses a system where the desired rate of cabin pressure change, d(P.sub.cd)/dt is computed as a function of a ratio DP.sub.c /DP.sub.a, where:
DP.sub.c =the remaining change in cabin pressure during either an ascent or descent; and PA1 DP.sub.a =the remaining change in ambient pressure during either an ascent or descent;
The '460 patent discloses controlling the rate of cabin pressure change and adjusting the rate so if the aircraft slows its ascent/descent rate, the rate of cabin pressure change will also slow. Similarly if the aircraft increases its ascent/descent rate, the rate of cabin pressure change will also increase. This ensures the aircraft fuselage does not exceed a predetermined maximum pressure differential between the cabin pressure and external ambient pressure.
However, the '460 patent uses a function generator (item 33, FIG. 1) having a single, nonadjustable operating line as a function of DP.sub.a to provide a set point for the desired rate of cabin pressure change, d(P.sub.cd)/dt. Setting the desired rate of cabin pressure change based upon this single operating line for an infinite number of possible cabin pressure ascent and descent profiles, leads to a sacrifice in the control of cabin pressure since the desired rate of cabin pressure change is constrained to take on values along the single nonadjustable operating line of the function generator.